Photonic crystal fibers (hereinafter referred to as PC fibers) have been known as an optical fiber which exhibits strong wavelength dispersion. The PC fiber includes a solid or hollow core which extends the center of the fiber in the longitudinal direction of the fiber and a cladding which surrounds the core and has a plurality of holes extending along the core. The cladding provides a photonic crystal structure which brings periodic changes in refractive index.
If the hollow core and the holes of the cladding of the PC fiber are left opened at an end face of the PC fiber, foreign objects may possibly come into the hollow core and the holes of the cladding, thereby impairing the optical properties. From this aspect, Japanese Unexamined Patent Publication No. 2002-323625 discloses a PC fiber whose end face has been fused to seal the hollow core and the holes of the cladding.
However, when the end face of the PC fiber is fused to seal the hollow core and the holes of the cladding, the length of the sealing portion of the fiber will be as long as about 1 mm. As a result, when signal light is transmitted directly to the fiber from a condenser or a light source, the signal light leaks out of the fiber to cause a problem of great connection loss.
Further, Japanese Unexamined Patent Publication No. 2002-323625 discloses that the length of the sealing portion should be determined such that almost no influence is caused on optical waveguiding. However, the Publication is unclear because of lack of specific descriptions of the length. Since the length of the sealing portion varies depending on the type of the fiber, it is necessary to determine the length of the sealing portion by repeating examinations of the connection loss on each fiber. As a result, working efficiency deteriorates and the resulting products vary in quality.